The present invention relates to methods of converting sugars to sugar alcohols using aqueous phase catalytic hydrogenation.
Conventional heterogeneous catalysis has usually involved petroleum processing; however, in recent years there has been an increased emphasis on aqueous systems. As opposed to petroleum- or hydrocarbon-based systems, water-based systems have less toxicity and fewer environmental problems. Additionally, aqueous systems are well-suited for biologically-produced feedstocks. For example, sugars from biological sources can be extracted with or produced in water. Then, to prepare sugar alcohols from these sugar solutions, it is economically necessary to conduct catalytic hydrogenation in the aqueous phase.
For the commercially important glucose to sorbitol hydrogenation, Gallezot et al. remarked that the challenge was to obtain a high conversion of glucose with a high selectivity to sorbitol and xe2x80x9ca high stability of the catalyst during a long period of time.xe2x80x9d See Gallezot et al., xe2x80x9cGlucose Hydrogenation on Ruthenium Catalysts in a Trickle-Bed Reactor.xe2x80x9d However, despite the work of Gallezot et al. and others, there remains a need for improved methods of aqueous phase hydrogenation of sugars to sugar alcohols.
In a first aspect, the invention provides a method of converting sugar to sugar alcohol by catalytic hydrogenation in the aqueous phase. In this method, an aqueous sugar solution is passed into a reaction chamber. Temperature of solution in the reaction chamber is maintained at less than 120xc2x0 C., and pressure in the reaction chamber is maintained at 100 to 3000 pounds per square inch gauge hydrogen gas overpressure. The reaction chamber contains a hydrothermally stable catalyst and, in the reaction chamber, the sugar reacts with hydrogen to produce a sugar alcohol. The reaction conditions are such that, when measured after 300 hours at the same reaction conditions, at least 97% of the sugar is converted to a sugar alcohol. Of course, each sugar is converted to its corresponding sugar alcohol, e.g. glucose to sorbitol, lactose to lactitol, etc. That the conversion of sugar to sugar alcohol is measured at 300 hours means that to test satisfactory reaction conditions (including selection of catalyst), a measurement is made after continuing to run 300 hours of operation at the same conditions without intervening steps of reactivating or replacing the catalyst; it does not mean that the invention is limited to reactions run for 300 hours or more.
In a second aspect, the invention provides a method of converting sugars to sugar alcohols by passing an aqueous sugar solution over a catalyst comprising ruthenium on a titania support, where the titania in the support is 75% or more in the rutile phase as measured by x-ray diffraction.
The aqueous sugar solutions used in the inventive methods contain at least one sugar dissolved in water. The sugar to be converted is preferably a monoxe2x80x94or disaccharide. Examples of preferred sugars that are hydrogenated in the present invention include: glucose, lactose, lactulose, fructose, erythrose, arabinose, mannose, xylose, galactose, and talose.
The aqueous sugar solutions are usually derived from biological sources, typically plants such as corn. The invention is defined in terms of converting a single type of sugar; however, the aqueous solutions may contain a mixture of sugars. Preferably, the sugar feedstocks are obtained pure or are purified prior to use in the reactorxe2x80x94impure feedstocks (often containing sulfur-containing species) may poison the catalyst, and thus require more frequent catalyst regeneration steps. Preferably, the sugar solutions are more than 99% by weight, more preferably 99.9%, water and sugar. Preferably the sugar is a single type of sugar such as glucose, arabinose, etc.
The sugar solutions are preferably 1 to 70 weight % sugar, more preferably 7 to 45 weight % sugar. The aqueous sugar solutions are preferably fed into the reaction chamber at fast rates, preferably fed into the reaction chamber at a rate of at least 0.5 kg sugar per liter of catalyst bed per hour, more preferably 0.9 kg sugar per liter of catalyst bed per hour, and still more preferably a rate of 1.2 kg/L/hr to 1.9 kg/L/hr.
Temperature in the reaction chamber is preferably maintained below 120xc2x0 C. Higher temperatures require too much energy and can result in poorer selectivities and can cause faster catalyst degradation. More preferably, the temperature is maintained in a range of 90 to 120xc2x0 C. Temperature is measured by placing a thermocouple in (or on) a catalyst bed in the reaction chamber. Pressure in the reaction chamber is preferably maintained in the range of 100 to 3000, more preferably 250 to 1900 pounds per square inch gauge. Pressure is generated by the hydrothermal conditions and is maintained in a desired range by hydrogen gas overpressure.
The hydrogenation catalyst must be an active hydrogenation catalyst and must also be stable in hydrothermal conditions. It has been discovered that a Ru on rutile catalyst exhibits exceptional properties for the aqueous phase hydrogenation of sugars to their corresponding sugar alcohols. It is believed that additional catalysts might be developed by routine testing utilizing the conditions and results described herein. Use of impure feed can poison the catalyst leading to loss of activity; the catalyst can be regenerated either by discontinuing reaction and hydrogen treatment or by switching to a purer feed solution.
Preferably, the catalyst that has an active metal on a titania support. The active metal preferably includes ruthenium and the titania is at least 75% rutile as measured by x-ray diffraction. Additionally, the catalyst is preferably essentially nickel-free and/or rhenium-free. It is desirable that catalyst metal be distributed over the surface of a support in a manner that maximizes surface area of the ruthenium. The metal preferably constitutes 0.1 to 10 weight % of the catalyst. Amounts of ruthenium above this range may not increase the catalyst""s activity, while amounts below this range can have undesirably low processing rates. More preferably, ruthenium constitutes 1 to 5 weight % of the catalyst, and still more preferably 2 to 3 weight %. In a preferred embodiment, the active metal consists essentially of pure ruthenium. The ruthenium preferably constitutes at least 95 weight percent of the active metal, more preferably more than 98%, and still more preferably more than 99.8%.
The catalyst is preferably essentially without nickel, that is, nickel does not make a significant contribution to the catalytic activity of the catalyst. Nickel is prone to dissolution in the aqueous phase processing conditions and may contaminate the product. This is especially a problem where a food-grade product is desired, for example in hydrogenating carbohydrates. Moreover, nickel in the product stream can also present a problem with waste disposal. Additive metals such as nickel can also present complications when disposing or recovering catalyst. Preferably, the catalyst contains less than 0.1 weight % nickel, more preferably, less than 0.01 weight %.
Rhenium is another metal that can present the problems discussed above for nickel. The catalyst is preferably essentially without rhenium. This means that the rhenium to ruthenium ratio in the catalyst is less than 1:20 by weight. Preferably, rhenium, if present at all, is present in less than 0.005 weight % of the catalyst. Similarly, the catalyst is preferably essentially without cobalt.
The metal, preferably ruthenium, is preferably disposed on a titania support. For optimum activity, the metal should exist in small particles on the surface of the support. The surface of the support typically includes not only the exterior surfaces but also interior surfaces of a porous support. The support may be in a variety of forms such as powder, pellets, honeycomb, etc. The titania is preferably composed of at least 75% rutile, more preferably at least 90% rutile, and still more preferably at least 95% rutile. For purposes of the present invention, the % rutile is measured as follows. A powdered sample of the support (or catalyst) is analyzed by x-ray diffraction using a copper x-ray source operating at 45 kV and 40 mA scanning over the range of 5 to 75 2-theta degrees. The rutile and anatase phases can be identified by comparison with the JCPDS database reference patterns. The peak height of the largest rutile peak and largest anatase peak are used for quantitation. The % rutile is determined as a percentage of its peak height divided by the sum of the heights of the largest rutile and largest anatase peaks.
Hydrogenation reactions are best controlled by using a pure titania support. The support is preferably at least 90 weight % titania, more preferably at least 99.5% titania. Because binders, such as clays, may dissolve or interfere with catalytic activity, the support preferably does not contain binders. The catalyst can also be characterized by elemental analysis; preferably the catalyst comprises 54 to 60 weight % titanium, and 36 to 40 weight % oxygen.
Although it is possible to generate a rutile support in situ by selection of processing conditions that favor the formation of rutile, better and more consistent activity and stability can be achieved by using a support with a high level of rutile (at least 75%, more preferably 90%, and still more preferably 95%) prior to depositing ruthenium on the support""s surface. Titania supports having a high level of rutile phase can be purchased or prepared. A suitable support is P25 code 7709 titania, available from Degussa Corporation, Parsippany, N.J., USA. This support can be used without additional calcination or thermal processing. Alternatively, titania can be prepared by known methods such as oxidation of titanium, water treatment of titanium chloride, and hydrolysis of titanium alkoxides. The support can be titania powder, but is preferably in the form of tablets, pellets, extrudates or other forms for use in a fixed bed catalyst system.
Ruthenium can be coprecipitated with titania, but for greater activity and economy it is preferably deposited onto the titania support. The ruthenium can be deposited onto a titania support by impregnating with aqueous ruthenium compositions such as aqueous ruthenium chloride. Other methods such as vapor deposition are also possible. After impregnation, water is removed by heating and the precipitated ruthenium compound reduced to the metal by reduction with hydrogen at elevated temperature. The reduction is preferably conducted at below 300xc2x0 C., since reductions above this temperature have been shown to reduce the titania resulting in migration of the titanium to the ruthenium causing less of the ruthenium surface to be exposed, and loss of catalytic activity.
The inventive method is not limited to particular reactor types, and may generally be conducted in continuous stirred tank reactor (CSTR), fixed bed, fluidized bed, expanded bed, etc. The reaction chamber is where the catalyst is situated and where the hydrogenation reaction occurs. In the present invention, the term xe2x80x9creaction chamberxe2x80x9d refers to the portion of the reaction chamber that is maintained at the reaction conditions. The volume of catalyst is defined based on the volume measured in the determination of the apparent bulk density of the catalyst, that is, as settle catalyst particles.
The amount of sugar converted to sugar alcohol is a function of sugar conversion and product selectivity. Preferably, the reaction converts at least 95% of the sugar in the feed stream, more preferably at least 97% and still more preferably at least 99%. Selectivity to the corresponding sugar alcohol, and percent of sugar converted to sugar alcohol, are each at least 95% more preferably at least 97% and still more preferably at least 99%. While measurements by liquid chromatography can be quite accurate (especially when averaged), gas chromatography is selected as the technique to measure the inventive methods, where nonvolatile components are derivatized prior to injection into the gas chromatograph.
One advantage of the inventive method is the capability to run the hydrogenation reaction over extended periods of time without regenerating the catalyst, and while maintaining excellent conversion and selectivity. The stability of the invention is measured by allowing the reaction to proceed for at least 300 hours, without regenerating the catalyst, and measuring the concentrations of sugar and sugar alcohol in the product stream. While the invention is measured according to this procedure, the inventive methods include production runs for less than and more than 300 hours. Preferably the reaction is conducted for at least 200 hours, more preferably at least 300 hours, and still more preferably at least 400 hours without regenerating the catalyst.
Another advantage of preferred embodiments of the invention is the extremely low levels of metal contamination in the product stream. Preferably the level of metals in the product stream is less than 15 parts per billion (ppb) in total metals (testing for metals present in the catalyst), more preferably less than 8 ppb.